ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        stack<int> s1, s2;
        while(l1)
        {
            s1.push(l1->val);
            l1 = l1->next;
        }
        while(l2)
        {
            s2.push(l2->val);
            l2 = l2->next;
        }
        int carry = 0;
        ListNode *ans = nullptr;

        while(!s1.empty() || !s2.empty() || carry != 0)
        {
            int a = s1.empty() ? 0 : s1.top();
            int b = s2.empty() ? 0 : s2.top();
            if(!s1.empty()) s1.pop();
            if(!s2.empty()) s2.pop();

            int cur = a + b + carry;
            carry = cur / 10;
            cur %= 10;

            auto curnode = new ListNode(cur);
            curnode->next = ans;
            ans = curnode;
        }
        return ans;
    }